I. Field of the Invention
This invention pertains to optical communication systems wherein wavelength division multiplexed (WDM) optical signals having multiple wavelengths are transmitted through an optical medium. More particularly, the present invention relates to a method and device for monitoring of channels in a WDM optical signal.
II. Description of Related Art
In WDM optical communications systems, a WDM optical signal includes a plurality of discrete optical wavelengths or channels which can be modulated with data and transmitted through an optical medium such as an optic fiber. The modulated channels may carry data, for example, to downstream optical network units (ONUs) and used for various purposes. In such WDM optical systems, it is desirable to monitor the channels to obtain system operational data. For example, it is important to know whether a particular channel is on or off, and thus available for carrying a data signal; whether a particular channel has a desired power level; and whether a particular channel is at an appropriate wavelength, e.g. to determine whether drift has occurred.
Several devices and techniques exist for monitoring channels in a WDM optical system. For example, an optical spectral analyzer can be employed to measure spectrum characteristics. However, the use of such a device has several drawbacks. For example, spectral analyzers contain moving parts and are, therefore, susceptible to frequent breakdown and repair. Moreover, the units themselves are expensive and multiple units are required, e.g. one at each add/drop node in the optical network.
Optical gratings can also be used to monitor WDM signal channels. Such gratings angularly disperse the individual wavelengths in a multiple wavelength optical signal from each other whereupon the dispersed wavelengths can be received by detectors for monitoring channel characteristics. The operation of such gratings is more fully described in U.S. patent application Ser. No. 08/690,696 filed on Jul. 31, 1996 and entitled "Attenuation Device For Wavelength Multiplexed Optical Fiber Communications".
Another technique for monitoring WDM signal channels employs a wavelength selective delay to convert wavelengths into the time domain. Such a technique utilizes a modulator to form a pulse from all of the wavelengths in the multiple wavelength signal carried by a transmission optical fiber so that the formed pulse is comprised of wavelengths in all of the channels. The pulse is then processed by a dispersive delay line such as a continuously dispersive line (e.g. a length of single mode fiber) or a discrete delay line wherein discrete delays are provided to different wavelengths (e.g. a series of fiber Bragg gratings, each being reflective at a different wavelength). The amount of dispersion required depends on how short a pulse can be generated by the modulator. Thus, a lower limit is placed on the expense of this technique. For example, a cheap modulator will generate long pulses which, in turn, will require an expensive, large dispersive delay line for processing.
Accordingly, there exists a need for providing for monitoring of channels in a WDM signal using relatively low amounts of dispersion and avoiding the need for a modulator to form an optical pulse.